st: splines with interactions

Dear Stata Users,
I'm modeling body growth using the -mkspline- command and I was looking
for some advice on how to include interactions between the splines and
gender. Specifically, if my model has 3 splines (age_1, age_2, age_3),
is it necessary, from a statistical perspective, to include a gender
interaction for the third spline (age_3*sex) even if only the gender
interaction with the second spline (age_2*sex) is significant. In other
words, does a model such as weight = age_1 + age_2 + age_3 + sex +
age_2*sex, violate any assumptions? Likewise, if the model had a
significant interaction with the first spline (age_1*sex) is it also
necessary to include both age_2*sex and age_3*sex, even if these are not
significant?